Evaluation of Anatomic Variations in Maxillary Sinus with the Aid of Cone Beam Computed Tomography (CBCT) in a Population in South of Iran.

STATEMENT OF THE PROBLEM
Anatomic variations of the maxillary sinus can be detected in cone-beam computed tomography (CBCT) and may assist to locate the posterior superior alveolar artery (PSAA) and define the maxillary sinus morphology more accurately for a more strict surgical treatment plan.


PURPOSE
The study aimed to determine normal variations of the maxillary sinus with the aid of CBCT in a sample population in south of Iran.


MATERIALS AND METHOD
This cross-sectional prevalence study was based on evaluation of 198 projection data of CBCT scans of some Iranian patients aged 18-45 who referred to a private oral and maxillofacial radiology center in Shiraz from 2011 to 2013. CBCT scans were taken and analyzed with NewTom VGi device and software. The anatomic variations which were evaluated in the axial images included the presence of alveolar pneumatization, anterior pneumatization, exostosis, and hypoplasia. Moreover the location and height of sinus septa, and the location of PSAA were assessed. SPSS software (version 17.0) was used to analyze the data.


RESULTS
In a total of 396 examined sinuses, maxillary sinus alveolar pneumatization was the most common anatomic variation detected. Anterior pneumatization was detected in 96 sinuses (24.2%). Antral septa were found in 180 sinuses (45.4%) and were mostly located in the anterior region. Meanwhile, PSAA was mostly detected intra-osseous in 242 sinuses (65.7%).


CONCLUSION
Anatomic variations of the maxillary sinus were common findings in CBCT of the maxilla. Preoperative imaging with CBCT seems to be very helpful for assessing the location of PSAA and the maxillary sinus morphology, which may be used to adjust the surgical treatment plan to yield more successful treatments.


Introduction
The maxillary sinus in adults is composed of a pyramidshaped cavity in the facial skull with its base at the lateral nasal wall and its apex extending up to the zygomatic process of the maxilla. [1] It can exhibit anatomic variations such as pneumatization, hypoplasia, antral septa, exostosis, and variations in location of the arteries. [2] All the surgical interventions in the posterior maxillary region require detailed knowledge of the maxillary sinus anatomy and possible anatomical variations. [3] Maxillary sinus hypoplasia (MSH) is the under-  [4] The narrow infundibular passage associated with the absence of a natural ostium would result in mucosal thickening of the hypoplastic sinus. [5] Furthermore, MSH causes the proximal extension of the lateral nasal wall and subsequently makes the surgical procedures difficult. [2,6] Maxillary sinus septa are barriers of cortical bone.
The shape is described as an inverted gothic arch arising from the inferior or lateral walls of the sinus that divide the maxillary sinus floor into multiple compartments, known as recesses. [1,3] These septa were first analyzed by Arthur S. Underwood, an anatomist who reported their prevalence and characteristics and these septa were afterwards, referred to as Underwood's septa. [1] In a systematic review published by Pommer et al., [7] electronic and hand searching of English literature were employed to identify the studies published from 1995 to 2011.They reported that the observed septa were at least 2-4 mm in height, and 7.5 mm on average. They were present in 28.4% of 8923 investigated sinuses (95% confidence interval: 24.3-32.5%). Septa were located in premolar, molar, and retromolar regions in 24.4%, 54.6% and 21.0% of cases, respectively. Their orientation was transverse in 87.6%, sagittal in 11.1%, and horizontal in 1.3% of the studied cases. Complete septa (dividing the sinus into two separate cavities) were found only in 0.3% of samples. Other rare conditions included multiple septa in one sinus (4.2%) and bilateral septa (17.2%). Moreover, the diagnosis of septa by using panoramic radiographs yielded incorrect results in 29% of cases. [7] Septum removal before sinus augmentation is a preferred procedure, as with the septum in place, there is a high possibility of membrane perforation that results in maxillary sinusitis. [3] Dental panoramic radiography, computed tomography (CT), and cone beam computed tomography (CBCT) have all been used to identify the maxillary sinus septa. [8][9][10][11][12][13][14][15] CBCT is a technique that has been proposed for maxillofacial imaging during the last decade and was first reported by Mozzo et al. [1,16] The posterior superior alveolar artery (PSAA) and infraorbital artery (IOA) are the branches of maxillary artery that supply the lateral sinus wall and the overlying membrane. The blood supply of the maxillary sinus and Schneiderian membrane comes from the maxillary artery. [17] The presence of this artery was first mentioned by Strong in 1934. [18] The branches of maxillary artery should be taken into consideration because of the potential risk of bleeding during the procedures such as open sinus lift surgery, horizontal osteotomy of the maxilla, Le Fort I fracture treatment, and Caldwell-Luc surgeries. [17][18][19][20] In a study done by Rahpeyma  Many imaging techniques such as panoramic, waters, Caldwell, CT, MRI, and CBCT can be used to study the maxillary sinuses region. For a long period, skull projections including Waters, Caldwell and lateral sinus were used for evaluation of the paranasal sinuses.
Waters view is useful for gross evaluation of the maxillary sinus especially for localized mucosal thickening along the sinus floor, generalized thickening of the mucosal lining around the entire wall of the sinus, and near-complete or complete radiopacification of the sinus. Plain films are no longer considered to be a part of the primary imaging modalities. At best, they give only an overview of the anatomy and underlying pathoses, as they are limited to display three-dimensional (3D) structures in a two-dimensional (2D) plane. CT and MR imaging have the advantage of being able to show fine anatomic details in serial topographic sections, and thus excluding the gross volume averaging which is a characteristic feature in plain films. In fact, in most cases, when a plain-film study shows the probable presence of the disease, a CT or MR imaging is consequently obtained. [21][22] CBCT uses a cone-or pyramidal-shaped beam to acquire multiple projections in only one rotation. On the other hand, multislice computed tomography (MSCT) employs fan-shaped beams rotating around the patient to acquire multiple image slices. [2,[23][24] CBCT may be recommended as a low-cost dosesparing technique compared with standard medical computed tomography scans (MDCT), though CBCT has slightly more radiation exposure than routine panoramic radiography for dentomaxillofacial imaging. [1,16,[25][26][27][28][29][30][31] The effective dose from a standard dental protocol scan with MDCT is 1.5 to 12.3 times greater than comparable medium-field of view dental CBCT scans according to International Commission on Radiological Protection (ICRP 2007). [16] Moreover, beamhardening artifacts due to dental materials (like amalgam and crowns) and implants are weaker at CBCT than at MSCT. [32] To minimize the risk of postoperative complications of maxillary sinus floor lift and other surgeries in this region, it is crucial to be familiar with different anatomic and pathologic findings in sinus. [1, 8-13, 29, 33] As the maxillary sinuses are significant anatomic structures in dental practice that their exact and definitive radiological assessment is necessary, and considering CBCT as an important diagnostic image modality in dentistry, the recognition of anatomic variations of the maxillary sinuses in CBCT is noteworthy. [1][2] Several studies have been performed on the prevalence of anatomic variations in different populations; however, our information is insufficient regarding the Iranian population. Therefore, the aim of the present study was to determine the maxillary sinus normal variations with the aid of CBCT in a sample of population resident in south of Iran.

Materials and Method
This cross-sectional prevalence study was based on evaluation of CBCT scans of some Iranian patients aged

Results
In a total of 198 CBCT images, 396 sinuses were evaluated in which 130 cases belonged to females (65.7%) and 68 to males (34.3%).
Antral septa were found in 180 sinuses (     The alveolar antral artery is an important anatomical structure within the lateral maxillary sinus wall. The presence of this artery was first mentioned in an article by Strong published in 1934. [18] According to Ilgüy et al. [  The range of septa prevalence was found to be 24- In a study performed in Iran with spiral CT-scan, the prevalence of at least one septum was 29.5%. [3] The results of the present study revealed that sinus septa  [55] Another study reported that the mean maxillary sinus volume in girls was larger than that in boys aged 4-9 in a Japanese population. [56] Therefore, the current study seems to be justified concerning these differences observed in different ethnics.
Investigating the prevalence of these important anatomical features in Iranian population, especially in different parts of the country would be helpful for young surgeons in this population.

Conclusion
The anatomic variations of maxillary sinus are common findings in CBCT of the maxilla. Since some of these conditions can modify the surgery planning to more specialized procedures, they are crucial to be recognized in dental practice. Inevitably, preoperative imaging with CBCT is helpful for assessing the location of the PSAA, maxillary sinus morphology, and normal variations which may be used to adjust the surgical treatment plan to yield more successful treatments.